Research On Modulation And Control Strategy Of Current-fed Dual-active-bridge DC/DC Converters

IIIDual-active-bridge DC/DC converter(DAB DC/DC converter)has the advantages of high efficiency,soft-switch,dual power direction and so on.It has a wide application prospect in renewable energy vehicles.The current-fed dual-active-bridge DC/DC converter(CF-DAB DC/DC converter)has low input-current ripple and direct current control capability.This topology is an ideal solution for DC/DC converter in renewable energy vehicles.However,the CF-DAB DC/DC converter still faces several challenging problems: voltage spike caused by transformer leakage inductance,circulation current,parameter compensation,etc.The existing modulation schemes of DAB DC/DC converters are mainly aimed at voltage-fed dual-active-bridge DC/DC converters.Therefore,to further improve the control performance of CF-DAB DC/DC converters and expand their applications,this paper takes this topology as the research object,mainly focusing on the optimization of modulation strategy and control method.The main research contents and achievements of this thesis are as follows:1.In view of the voltage spike problem caused by transformer leakage inductance,this paper proposes a decoupled dual pulse-width-modulation strategy(decoupled dual-PWM).Without the need for additional auxiliary circuit,the dual-PWM strategy can pre-charge the leakage inductance of the transformer properly,thus avoiding the voltage spike problem caused by the leakage inductance of the transformer.In light load operation,the dual-PWM strategy can effectively inhibit the circulation current flow on the side of the current source,so as to improve the efficiency of the converter in the wide load range.The dual-PWM strategy can flexibly adjust the turn-on moment of the voltage-fed side switches.Compared with the existing modulation strategies,the dual-PWM strategy can effectively reduce the peak current of the leakage inductor,the circulation current and corresponding losses.At the same time,the decoupled dualPWM strategy avoids the coupling between the duty-cycle on the current-fed side and the duty-cycle on the voltage-fed side,and realizes the decoupling between the boost ratio,the leakage inductance of the transformer and the load conditions.This will simplify the design of the converter control loop.This chapter also introduces the design process,power consumption analysis and implementation of the modulation strategy in detail.This chapter analyzes the influence of parameter deviation of input inductance and leakage inductance of transformer.Aiming at the modulation strategy proposed in this chapter,this chapter realizes digital control based on TI TMS320F28335 DSP,and builds the principle prototype without auxiliary circuit.In this chapter,the effectiveness of the proposed modulation strategy is verified by further experiments.2.The problem of inductance parameter compensation for current-fed dual-active-bridge converters is still lacking in research.The inductance parameter compensation problem mainly revolves around the input inductance and the transformer leakage inductance.Aiming at the problem of parameter difference and uncertainty of transformer leakage inductance,a new parameter compensation method of transformer leakage inductance is proposed in this part.This method does not require additional high precision current sensors to measure the instantaneous current passing through the leakage inductance of the transformer.Aiming at the parameter difference of transformer leakage inductance,this method provides the reflected output voltage which can be adjusted flexibly and separately according to the parameter difference of transformer leakage inductance between different submodules.To solve the problem of parameter deviation(uncertainty)of transformer leakage inductance,this method can compensate based on parameter estimation to further improve the closed-loop control stability of converter and realize the suppression of current source cell side circulation and the minimization of current stress on switching devices.In addition,this part of the work also for the input inductance parameter deviation(uncertainty)in-depth analysis and research,through duty cycle predictive control to solve the problem.Aiming at the parameter compensation method proposed in this part of work,a prototype of current source type dual-active bridge DC converter with parallel input is built in this paper.Through further experiments,the effectiveness of the proposed parameter compensation method is verified.3.At present,the applications of three-port CF-DAB DC/DC converters are limited.The main reason is voltage spike caused by the leakage inductance.To solve this problem,a modulation strategy for threeport CF-DAB DC/DC converter is proposed in this paper.The modulation strategy can realize the advance commutation of the leakage inductance current of the transformer,the soft switching operation and the minimized circulation current.A principle prototype of a magnetically coupled three-port CF-DAB DC/DC converters is built in this paper,and the effectiveness of the proposed modulation strategy is verified by experiments.4.The pulsating-DC modulation strategy can minimize the capacitance of the DC busbar so as to make it possible to use film capacitors instead of electrolytic capacitors.The pulsating-DC modulation strategy also enables the inverter to achieve a lower equivalent switching frequency and a higher DC voltage utilization.In this part,the applications of CF-DAB DC/DC converter with pulsating-DC busbar are studied.This part of the work proposes a modulation strategy for CF-DAB DC/DC converter with pulsating output voltage.Even if the DC bus voltage is continuously pulsating,this modulation strategy can simultaneously realize the advance commutation of the current of transformer leakage inductance,the soft switching operation and the minimization of circulation current.In addition,the modulation strategy is also applicable when the load conditions change.A principle prototype of the CF-DAB DC/DC converter with pulsating output voltage is built in this paper,and the effectiveness of the proposed modulation strategy is verified through further experiments.